1. Technical Field of the Invention
The invention relates to a screening machine comprising two screening frames which are to be caused to oscillate and on which there is arranged an elastic screening lining and which are to be caused to oscillate by at least one oscillating drive.
2. Discussion of the Related Art
A known screening machine of that kind comprises two screening frames which oscillate relative to each other, each thereof having a plurality of transverse bearers to which an elastic screening lining is fixed. In relation to the central position of the screening frames relative to each other, the screening lining is longer than would correspond to the length of the frames so that the screening lining hangs down loosely between the individual transverse bearers. The entire screening lining therefore forms a plurality of screening web portions disposed in succession, which extend transversely with respect to the longitudinal direction of the screening machine, each screening web portion being disposed between two adjacent transverse bearers.
When the screening frames are at their one point of reversal of movement, some screening web portions of the screening lining are tensioned while the other screening web portions which are between the tensioned screening web portions hang down loosely between each two adjacent transverse bearers. At the other point of reversal of movement of the screening frames the tensioned condition of the screening web portions is reversed so that the screening web portions which have been previously tensioned now hang down between two respective adjacent transverse bearers and the screening web portions between them, which previously were loose, are now tensioned. Therefore the tensioned screening web portions and the sagging screen web portions alternate from one point of reversal of movement of the screening frames or the oscillating drives of the screening machine, to the other.
The two frames of the screening machine are caused to oscillate by mutually independent unbalanced drives in such a way that the two frames oscillate in 180.degree. phase-shifted relationship with each other so that they are in a condition of mass equilibrium. The frames both oscillate in their longitudinal direciton, that is to say in one dimension in the same direction and in a common plane.
Another known screen machine has only a single screening box accommodating a plurality of transverse bearers to which the screening lining is secured. Provided at each of the transverse bearers is an unbalanced drive by which the associated transverse bearer is reciprocated, that is to say caused to oscillate, in the longitudinal direction of the screening frame. In this arangement the oscillations are generated in such a way that respective adjacent transverse bearers oscillate in 180.degree. phase-shifted relationship so that those transverse bearers are also in a condition of mass equilibrium. The unbalanced drives for the transverse bearers are synchronized more specifically either mechanically by a chain drive or electrically by a synchronized circuit. As a result the screening machine is in a condition of mass equilibrium and does not apply any reaction forces to the place of mounting or the foundation structure.
These known screening machines suffer from a particular difficulty in that the movable parts of the screening machine, that is to say in particular the screening lining, must be sealed relative to the stationary parts of the screening machine which are adjacent thereto, in particular relative to the two side walls. That sealing effect is difficult to achieve and gives rise to particular wear and operational problems.